Toward Point-of-Care Drug Testing: A New Paradigm for On-Chip Chromatography Coupled with Surface-Enhanced Raman Scattering PROJECT SUMMARY In principle, surface enhanced Raman spectroscopy (SERS) can provide unique identification to a target analyte, especially small molecules such as illicit drugs. In practice, however, this is not always true for real- world samples due to various forms of interference such as nonspecific binding from large biomolecules (proteins, DNAs, etc.) in complex biological samples to the surface of metallic nanoparticles (NPs). Therefore, a selective (e.g., assay) or separation (e.g., chromatography) process with certain specificity is needed to transform existing SERS technique from a lab analysis tool into clinical usage. One urgent need for such on- chip sensing technology is point-of-care (POC) drug testing due to the alarming spread in drug abuse, fuelled by the production of inexpensive, easy-to-manufacture drug variants. Existing testing methods, including gas chromatography with mass spectrometry (GC/MS) and high-performance liquid chromatography (HPLC), demand tedious sample preparation with expensive reagents and substantive operator expertise, and are ill- suited to clinical and law enforcement settings for POC testing. Diatomaceous earth, also known as diatomite, consists of fossilized remains of diatoms, a type of hard-shelled algae. As a type of abundant and cost-effective natural photonic biosilica from geological deposits, it has a variety of unique properties including highly porous micro- and nano-structures, excellent adsorption capacity, rich surface chemistry, and unique micro-fluidic properties. The objective of this R21 project is to develop a new on-chip chromatography technology coupled with SERS sensing using diatomaceous earth thin film, which can provide a portable, cost-effective solution to detect various illicit drugs in complex biological fluidic samples. Particularly in this project, we will apply this lab-on-chip technology for POC testing of cocaine and Marijuana. Briefly, we will spin coat ultra-thin (eventually down to monolayer around 5?m) diatomaceous earth on regular glass substrates, and integrate high-density plasmonic NPs through in-situ synthesis. This hybrid photonic-plasmonic nanostructured thin film will act simultaneously as a thin layer chromatography (TLC) to separate drug molecules from the complex biofluid and ultra-sensitive SERS substrates to probe the signature Raman peaks. Through the combination with commercially available portable Raman spectrometers, our diatomite TLC-SERS technology will enable rapid, multiplex POC sensing with high specificity. Compared with existing TLC-SERS methods based on silica gel plates, we expect our technology to achieve >100 enhanced sensitivity (down to <1ppm) due to the photonic crystal effect from diatom frustules and the ultra-small thickness of the multi-scale porous structure. Last but not the least, the wide availability and low cost (<$1/chip) of the multifunctional TLC-SERS substrates are perfectly suitable for POC drug testing of cocaine, marijuana, and other illicit drugs from urine, saliva and blood. We must point out that such TLC-SERS sensors can be readily applied for label-free sensing for many other applications such as food safety, water quality monitoring, and allergic biomarker detection.